Generate multiple specimens when multiple families are supplied

[RoelN]

Instead of generating a single specimen, allow the build process to take multiple directories of fonts and build a single specimen for each one.

Combined with a --watch parameter, relevant specimens would auto-update whenever one of their fonts changes. So you'd always have an up-to-date "specimen archive"

localhost:1234 Homepage with links to all fonts
localhost:1234/recursive Recursive Variable Specimen Page
localhost:1234/bungee Bungee Specimen Page
etc.